In applications that use wavelength or frequency sweeps of lasers, there are three characteristics needed to receive accurate information about the application: (1) the continuity and monotonicity of the wavelength sweep versus time; (2) the linearity of the sweep versus time; and (3) the ability to maintain continuity, monotonicity and linearity over time and environmental changes (such as temperature, humidity). For example, the shape of a molecular gas absorption feature may be distorted by a discontinuity—a forward or backward jump—in the wavelength sweep of the laser. In another example, wavelength discontinuities can reduce the quality of an OCT image of tissue. Thus, it is desirable to eliminate wavelength discontinuities from swept-wavelength lasers.
FIG. 1A illustrates a continuous linear frequency sweep of an ideal laser over time. FIG. 1B illustrates a continuous, but non-linear frequency sweep of an exemplary laser over time. FIG. 1C illustrates a discontinuous frequency sweep of an exemplary laser over time.
Wavelength discontinuities in single-mode, swept-wavelength lasers are often caused by sudden transitions from one single-longitudinal mode of the laser to another single-longitudinal mode, where the wavelengths of the modes are substantially different. These transitions are generally referred to as mode hops. Wavelength discontinuities may also occur in multi-mode, swept-wavelength lasers such as a Fourier Domain Mode Locked (FDML) laser and others when a sudden transition occurs between a group of modes of the laser to another group of modes of the laser, when the average wavelengths of the two groups are substantially different.
Even when the sweep is continuous and monotonic, in many applications it is advantageous for the laser wavelength to follow a specific temporal profile. For example, in OCT a sweep versus time that is linear in the optical frequency versus time is preferable to enable Fourier post-processing of the sweep data. In other applications, for example, OCT or telecommunications testing, it is advantageous to sweep the optical frequency versus time non-linearly versus time to compensate for other effects in a device or material under test.
Attempts in the prior art to reduce or eliminate mode hops and control the sweep profile are numerous, but generally unsatisfactory or temporary. Although it may be possible to carefully remove all discontinuities and non-linearity at a point in time, with the passage of time or changes in (for example) temperature will then create additional discontinuities and non-linearities. For example, external cavity lasers operate in near continuous single-mode using an external cavity mechanism coupled with a gain medium. Mode hops are prevented through accurate, tightly-toleranced components and precision alignment of the cavity, or using real-time elements such as piezoelectric transducers that adjust the cavity length in real-time.
Other laser configurations use an intra-cavity element. Over time, the alignment of the laser degrades or the components wear, which may cause mode hops and changes in the sweep profile versus time. As the ambient temperature or pressure change, the alignment can degrade, which can also cause mode hops and changes in the sweep profile versus time. Vibrations external to the laser or vibrations internal to the laser (due to high-speed mechanical operation) may also misalign the cavity, which again may cause mode hops and changes in the sweep profile versus time.
Another challenge is that mode hops may occur anywhere in the wavelength sweep of the laser with a mechanical tuning mechanism. Hence, there is a need to monitor the entire sweep for signs of wavelength discontinuities and changes in the sweep profile of a laser, and to correct the laser sweep for these changes.
Another class of single-mode laser for producing swept wavelengths is monolithically-constructed semiconductor lasers. Monolithic semiconductor lasers include several sections or segments in the semiconductor, which serve as adjustable cavity mirrors, laser gain, cavity phase and (optionally) external amplification. Examples are Vertical Cavity Surface Emitting Lasers (VCSELs), VCSELs with Micro-electromechanical systems (MEMS) tuning structures, Sampled Grating Distributed Bragg Reflector (SGDBR) lasers, Super-Structure Grating Distributed Bragg Reflector (SSGDBR) lasers and similar devices. Because these lasers are monolithic with no moving parts, their cavities are extremely stable and can operate in single-longitudinal mode with narrow linewidth and long coherence length. Tunable semiconductor lasers of this class require multiple laser current signals to tune the wavelength, presenting a challenge to creating wavelength sweeps without wavelength discontinuities. There is a need for an apparatus and method for controlling monolithic semiconductor lasers, reducing or eliminating mode hops within their wavelength sweeps and controlling their sweep profiles of wavelength versus time (to create, for example, linearity).
Monolithic tunable lasers with integrated gratings to enable tuning are now common in telecommunications applications. These tunable lasers are unique in that they offer wide wavelength tuning (for example 1520 to 1565 nanometers) and the capability for fast wavelength tuning (sweeps in microseconds) all on the same monolithic chip. Such a tunable laser will be referred to herein as a Semiconductor Monolithic Tunable Laser Source (SMTLS).
SMTLSs have wavelength regions associated with combinations of Back Mirror Drive and Front Mirror Drive. The complexity of the wavelength versus current map is shown in the FIG. 2. SMTLS devices were defined and developed to allow one of an array of specific wavelengths to be output, for example allowing selection of any one of the standard (ITU) wavelengths.
Work has been done in the past to allow fast switching of an SMTLS laser from one wavelength to another wavelength, anywhere in the tuning range of the laser, as discussed in U.S. Patent Application Publication 2009/0059972. Prior art involves using knowledge of the initial and final wavelength, and the currents associated with each wavelength, as well as feedback control systems to quickly lock the laser to its destination wavelength. This methodology is useful for applications in telecommunications that require discrete changes from one wavelength to another, destination wavelength.